A straight line is a one-dimensional geometry with no fixed ends.The general equation (linear equation) of first degree in x and y is, $$Ax+By+C=0$$

The tangent is not initiated from circle only but infact from many geometrical figures and it is very inportant to study .

The two sides of the line mean the side containing origin and not containing origin.If linesAx1+By1+C andAx2+By2+C have same sign, then P and Q lies on the same side of the lineAx+By+C=0.If linesAx1+By1+C andAx2+By2+C have opposite sign,then P and Q lies on the opposite side of the lineAx+By+C=0.

Any two lines through the Origin may be written as y = mx and y = tx where m and t are their gradients(slopes). So (y - mx)(y - tx) = 0 giving y - mx or y - tx = 0 must represent the pair. The homogeneous equation of second degree in x and y is given by, $$ax^2+2hxy+by^2=0